Is it a duffle bag? A backpack? Both? Whatever the answer, the Ignoble Lenore Capsule Backpack ($250) is definitely handy. Made from ballistic nylon with a Ripstop nylon interior, the Lenore features padded shoulder straps and an airmesh back for those times when you want a backpack, and a padded exterior handle for when you prefer a duffle. Other features include dual exterior zippered "stash" pockets, a dual-zippered main compartment with an internal book sleeve and hanging pocket, side compression straps, and a no-nonsense, military-esque look.
Just because you're carrying several pounds of gear doesn't mean your bag has to weigh a nearly equal amount. The North Face Flyweight Rucksack ($50) weighs less than 13 oz. thanks to a lightweight ripstop nylon build and can be stuffed completely into an interior pocket when not in use, yet can still handle up to 25 liters of stuff, organized with various pockets and straps, and carried on your happy shoulders by dual Airmesh straps. Pack heavy, travel light.